Process of producing a phosphate coating on metals high in aluminum



Patented Mar. 14, 1950 PRocEss or PRODUCING A PHOSPHATE CGATINGr NMETALS HIGH -INL ALU MIN UM Robert 0. Gibson, Birmingham, and Willia'mS. Russell, Royal Oak; Mich., assignors to Parker Rust Proof Company,Detroit, Mich."

No Drawing. Applicationlvlay 22, 1947,

SerialNo. 749,868 r 3 ClaimS. (Cl. 1486.15)

where the production is for a considerable time, 7

mainly on aluminum surfaces, the operation of the'solution is markedlyimproved by introducing iron into the. solution. In many factories it ispreferred to apply the coating solution by the spra"ngi process. When aphosphate coating solution" containingiro'fi is sprayed through the air,the ferrous phosphate inthe solution is largely oxidized to an insolubleferric phosphate and precipitated as sludge. Therefore, it isimpractical to maintain or improve the coating action of the solution bythe addition of iron where the solution is applied by spraying.

Where'the coating for a considerable time is entirely or predominatelyof aluminum surfaces,

the difficulty referred to in our prior application is encountered whenthe solution is applied by spraying-as well as'when employed by theimmersion process. A great deal of difilculty has been encountered in anendeavor to remedy this situation. At last it has'been discovered thatthe coating action of a solution of the type disclosed in our aforesaidprior application can be maintained where the solution is applied byspraying to predominately aluminum surfaces by the addition of fluorinecompounds such as sodium fluoride and neutralizing sodium compounds suchas sodium carbonate. Where these remedial compounds are added to thesolution there is formed an insoluble compound apparently having thecomposition indicated by NasAlFe, and this insoluble material isprecipitated, removing the excess aluminum from the solution. Asdisclosed in our said prior application, one example of the generalpurpose solution for preparing a coating bath which has been usedsuccessfully follows:

I Pounds H3PO4, 75% 37o HNOs, 42 B 715 ZnO 425 Boric acid 180 165 Sodiumbifluoride s 170 Water to test 46.5 136. at 80 F.

When 60 grains of the solution are diluted to 1000 00., from 2.4 to 2.6cc. of tenth normalNaOI-I should be required to neutralize logo. of thesolution using brophenol blue as indicator and from 19.5 to 20.5 of thehydroxide using phenolphthalein as indicator. ZnO or HsPOlis added ifnecessary to bring the solution within that range. A solution made up inthis way may be sprayed upon the work, or articles to be coated may bedipped in the solution.

A general purpose replenishing solution for the bath prepared asindicated above maybe made up as follows:

. Pound %M1I3PO4 1,295 HNO3,42 Be 700 ZnO 745 H2130: i 235 60% HF 450NazCOs 25 Water to test 460 Be. at F.

The foregoing ingredients are believed to furnish in the solution thefollowing compounds in the proportions given below.

. Per cent Zn' ('BFDz s 7.8- Zn (NO3)2 13.8 Z1l2 -18."7 H3303 -05 H3PO44.7 Sufiicient H3PO4 should be present to keep the bonate employed maybe proportionately reduced and as previously indicated, these additionalmaterials are not necessary at all where the proportion of aluminumcoated is suinciently reduced.

Where it is intended to employ the solution continuously upon work thesurfaces of which are almost entirely aluminum, the replenishingsolution may have fluoride added thereto, so that only the sodiumcarbonate needs to be added separately. Under such circumstances, areplenishing solution may be made substantially in accordance With thefollowing formula.

Parts by weight 75% H3PO4 1,295 42 B. HNO3' 700 ZnO 830 H3303 -1 730 60%HF 1,018 NaHFz 307 Water to produce a specific gravity of about Onepound of sodium carbonate or its equivalent should be added for each sixpounds of the modified replenishing solution.

Where mixed metals are coated, and particularly where the proportion ofaluminum to other surfaces coated varies materially from time to time,it is advisable to employ a replenishing solution substantially inaccordance with the first formula therefor given above, and to add bothfluoride and neutralizing agent when and as required.

It will be seen that the improvement consists in adding to the solution,materials which can combine with the excess aluminum and form insolublesodium aluminum fluoride and thereby remove the excess aluminum andstill leave sufficient fluoride compounds in the solution to accomplishits purpose. It will be readily understood that while exact formulashave been given which have proven satisfactory in commercial production,various modifications may be made in accordance with the .principlesknown to those skilled in the art.

In the appended claims in balance is used in reference to the properportion of free acid and total acid, as measured by the use ofbromophenol blue and phenolphthalein or equivalent indicators, thatbeing the usual meaning of the term in this art, as will be found inParker Rust Proof Co. v. Ford, 6 F. (2d) 649 (see especially 2nd columnon page 652) and in numerous patents issued since the time of thatdecision.

What We claim is:

1. A method of coating metal surfaces with a phosphate coating wheresaid surIaces consist predominantly of aluminum which comprises sprayingon to said surfaces a solution contaming acid phosphate and fluoborateand an oxidizing agent as its essential active ingredients andmaintaining the balance of the solution by adding a neutralizing sodiumcompound and acidic fluoride in amounts to combine with aluminum andform NasAlFe and remove excess aluminum from the solution.

2. A method of coating metal surfaces with a phosphate coating wheresaid surfaces consist predominantly of aluminum which comprises sprayingon to said surraces a solution containing acid phosphate and fluoborateand an oxidizing agent as its essential active ingredients andmaintaining the balance of the solution by adding a neutralizing sodiumcompound and acidic fluoride in amounts to combine with aluminum andform NasAlFs and remove excess aluminum from the solution and increasingand decreasing the amount of such additions as the proportion ofaluminum to other metals treated is increased and decreased.

3. A concentrated solution for use in replenishing a dilute solution forcoating surfaces consisting predominantly of aluminum, said concentratedsolution having a composition substan-' tially the same as thatresulting from dissolving in water the following ingredients:

Parts by weight 75% I-I3PO4 1,295 42 B. HNO3 700 ZnO 830 H2303 730 HF1,018 NaHFz 307 ROBERT C. -GIBSON.. WILLIAM S. RUSSELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. A METHOD OF COATING METAL SURFACES WITH A PHOSPHATE COATING WHERESAID SURFACES CONSIST PREDOMINANTLY OF ALUMINUM WHICH COMPRISES SPRAYINGON TO SAID SURFACES A SOLUTION CONTAINING ACID PHOSPHATE AND FLUOBORATEAND AN OXIDIZING AGENT AS ITS ESSENTIAL ACTIVE INGREDIENTS ANDMAINTAINING THE BALANCE OF THE SOLUTION BY ADDING A NEUTRALIZING SODIUMCOMPOUND AND ACIDIC FLUORIDE IN AMOUNTS TO COMBINE WITH ALUMINUM ANDFORM NA3ALF6 AND REMOVE EXCESS ALUMINUM FROM THE SOLUTION.